An important issue in IS-95 CDMA cellular and Personal Communications Service (PCS) wireless communication systems involves frequency handoffs between adjacent cells or antenna sectors. Each of the cells in such a system generally includes a base station, and the base station associated with a given cell may include an omnidirectional antenna or a multiple-sector directional antenna for communicating with mobile stations such as portable telephone handsets. As a mobile station moves throughout the system, its position relative to the system base stations changes, such that an ongoing call or other communication may need to be handed off from one base station to another, or from one antenna sector to another. Adjacent base stations and antenna sectors are typically configured to utilize different communication frequencies in order to minimize effects such as co-channel interference. Handoffs from one cell or sector to another may therefore involve changing the communication channel frequency from a current frequency to a new frequency. Such handoffs are generally referred to as inter-frequency or other-frequency handoffs.
A number of techniques have been proposed for improving the efficiency and success rate of inter-frequency handoffs in IS-95 CDMA systems. These techniques utilize messages such as Extended Handoff Direction Messages (EHDMs), Other Frequency Neighbor List Messages (OFNLMs), Other Frequency Neighbor List Response Messages (OFNLRMs) and Other Frequency Report Messages (OFRMs) to implement inter-frequency handoffs. Although proposed techniques based on these messages can reduce the number of call drops during a frequency transition, these techniques still suffer from a number of drawbacks. For example, the OFRM message is presently configured to allow a mobile station to report a signal-to-noise measure which is both "interference limited" in that it will typically decrease as the mobile moves across same-frequency cell boundaries, and "noise limited" in that it will also decrease as the mobile moves across other-frequency cell boundaries. For a measure which is interference limited, interference due to signals generated by other cells is greater than the noise level, while for a measure which is noise limited, the noise level is greater than the interference due to signals generated by other cells. A same-frequency cell boundary may be defined by a set of points at which the strength of a pilot signal from one cell exceeds that of a pilot signal from an adjacent cell, where both the pilot signals are at the same frequency. An other-frequency cell boundary may be defined as a set of points at which a signal from one cell at a designated frequency exceeds the strength of a signal from an adjacent cell at another frequency by a specified number of decibels. An OFRM message which utilizes conventional signal-to-noise measures cannot be used to distinguish same-frequency cell boundaries from other-frequency cell boundaries, and therefore does not provide an optimal trigger for inter-frequency handoffs.
In addition, the above-noted message-based techniques will often involve a base station commanding a mobile station to perform a periodic search for a new frequency as soon as the mobile station enters into a transition area near the edge of a new cell or sector. However, this periodic search tends to degrade voice quality of an ongoing call, while also reducing the speed of the search for new potential base stations at the current frequency. Moreover, in many practical applications, this periodic search for a new frequency can be unnecessary if the mobile is operating under certain types of radio frequency (RF) conditions. Yet another significant problem with the above-noted techniques is that the techniques may increase the likelihood of"ping-ponging" or rapid switching between the new frequency and the current frequency. More particularly, it may be possible in some areas of the system that both the new frequency and the current frequency will have good RF coverage, which could lead to ping-ponging if, for example, the mobile station reports the received power and signal-to-noise measure for only the new frequency.